1. Field of Invention
The invention relates to a device comprising a flat susceptor rotating parallel to a reference surface about a shaft perpendicular to this surface and comprising means for obtaining the stability of the susceptor held in sustentation and means for obtaining and measuring its rotary movement.
The invention further relates to a reactor chamber for vapor-phase epitaxy or chemical-vapor deposition provided with such a device. The invention can be used in the manufacture of flat rotating sample carriers for reactor chambers for vapor-phase epitaxy or chemical-vapor deposition, more particularly for the epitaxial growth from the vapor phase of layers of compounds of the III-V or II-VI group for forming semiconductor devices.
2. Description of Prior Art
The use of rotating sample carriers in the field of epitaxial growth from the vapor phase of the compounds of the III-V group is known from the publication entitled "Multi-Wafer Growth of Extremely Uniform GaAs Layers by Organometallic Vapor Phase Epitaxy" from the Electronic Materials Conference, University of Colorado, Jun. 19-21, 1985, by S. Komeno, H. Tanaka, I. Itoh et al. This publication shows that it is important during the growth of epitaxial layers from the vapor phase to rotate the sample in order to obtain uniform layers and to form components of high quality, such as field effect transistors, semiconductor diode lasers and many others.
Vapor-phase epitaxy imposes stringent requirements on the materials used within the reactor chamber. High temperatures of the order of 900.degree. C. are encountered routinely. High purity environment that is as particle-free as possible is required for the growth of high-quality and low-defect-density epitaxial layers. Highly-reactive and acutely-toxic gas ambients within the reactor chamber dictate the use of inert materials and leak-proof seals. The thermal variations across a susceptor, the difference in rotation rates of different susceptors, the rotation-rate variations from growth to growth, and the gas-flow disturbances and inhomogeneities must be kept to an absolute minimum to obtain highly-uniform epitaxial layers suitable for mass-production.
Susceptor rotation by external motor with mechanical feedthroughs is met with difficulties. The mechanical members traversing the reactor-chamber wall enclosing the acutely-toxic gases require leak-proof mechanical seals operating at high temperatures. Extremely complicated mechanical gearing mechanism for rotating susceptors in the planetary form is difficult to implement, especially when using the non-lubricated refractory materials suitable for the high-temperature, highly-reactive, and acutely-toxic gas ambients.
Susceptor rotation by gas flow in U.S. Pat. No. 4,860,687 to Frijlink, 1989 Aug. 29, was a significant improvement over rotation by external motor with mechanical feedthroughs. Two commercially-available devices are illustrated in two prior-art drawings of FIGS. 1a and 1b. These gas-flow rotation devices eliminated the need for mechanical feedthroughs and high-temperature mechanical seals. A viable planetary form of susceptor rotation was also demonstrated.
In FIG. 1a, a susceptor 14 rotates parallel to a reference surface 30 about a rotary shaft 20 perpendicular to reference surface 30. Susceptor 14 is held in sustentation and caused to rotate in the indicated direction by the action of gas flow through several gas inlets 40a, 40b, and 40c into several helical grooves 51a, 51b, and 51c on reference surface 30. The rotary movement is obtained by a force of viscosity of the gas.
FIG. 1b illustrates the prior-art planetary form of susceptor rotation by gas flow. A main susceptor 14 rotates in the indicated direction. Main susceptor 14 also includes a number of secondary susceptors 114, 214, and 314 on a reference surface 130 facing away from reference surface 30. Secondary susceptor 114 is used as an example applicable to secondary susceptors 214 and 314. Secondary susceptor 114 is held in sustentation and caused to rotate in the indicated direction by the action of gas flow through an additional gas inlet 42 in reference surface 30, a system of gas conduits 43, and several gas inlets 140a, 140b, and 140c into several helical grooves 151a, 151b, and 151c on reference surface 130. Similarly, the rotary movement is obtained by a force of viscosity of the gas.
Despite its significant improvement over rotation by external motor with mechanical feedthroughs, susceptor rotation by gas flow still suffers from many disadvantages as listed below:
(a) The allowable range of rotation rates is limited by the design of grooves 51a, 51b, and 51c, and the available gas-flow range. The minimum rotation rate is constrained by the minimum gas flow required for susceptor sustentation. The maximum rotation rate is constrained by the increase in the gap between the undersurface of susceptor 14 and reference surface 30 with increasing gas flow, where more and more gas flow bypasses grooves 51a, 51b, and 51c entirely, resulting in little or no increase in the rotation rate. PA1 (b) High rotation rates require high gas-flow rates. The large amounts of escaping gases from the gas-flow rotation schemes disrupt significantly the flow of gases in a reactor chamber, causing inhomogeneity and turbulence in the gas flow over the susceptor, resulting in inhomogeneous and defective epitaxial-growth layers. PA1 (c) Rotation rates of susceptors 14, 114, 214, and 314 are difficult to measure in a coated reactor chamber, and are sensitive to the individual machining tolerances of gas inlets 40a, 40b, 40c, 140a, 140b, and 140c, system of gas conduits 43, and grooves 51a, 51b, 51c, 151a, 151b, and 151c. Perfect matching of rotation rates between individual secondary susceptors 114, 214, and 314 in the planetary device is extremely difficult, because the individual rotation rates are not adjustable. PA1 (d) The failure or rate change in the susceptor rotation due to clogging in gas inlets 40a, 40b, 40c, 42, 140a, 140b, or 140c, or in system of gas conduits 43, or due to friction-producing dust particles are not directly detectable except by visual inspection, which is quite difficult to perform with heavily-coated reactor-chamber walls. PA1 (e) The process of starting and stopping rotation generates contaminating particles due to the frictional contact between rotating susceptor 14 and reference surface 30 when the gas-flow inputs start or stop. PA1 (f) Gap between the undersurface of susceptor 14 and reference surface 30 varies with rotation-rate changes. Increasing the rotation rate requires gas-flow increase, which increases the gap. The changes in the gap cause growth temperature variations and produce undesirable effect on epitaxial growth. PA1 (g) At constant mass-flow rates of gas input, the rotation rates of susceptors 14, 114, 214, and 314 vary with the growth temperature and pressure due to changes in the gas viscosity and volume. The calibration of gas-flow rates using each combination of growth temperature, reactor chamber pressure, and rotation rates is required. PA1 (h) Synchronization of susceptor rotation with source-gas switching during epitaxial growth is extremely difficult. Not synchronizing rotation with gas switching results in inhomogeneities in epitaxial-growth layers across the susceptor. This is especially critical for thin layers with layer thicknesses comparable to atomic dimensions. PA1 (i) Angular orientation and position of each susceptor 14, 114, 214, or 314 are not directly detectable except by visual inspection. In an automated production machine, the unsupervised loading and unloading of indexed wafers in a reactor chamber require machine-accessible data on the orientations and positions of individual susceptors. PA1 (a) No constraint on minimum rotation rate. The maximum rotation rate is mainly constrained by the design of the magnetic structures and the control electronics. PA1 (b) Escaping gases that disturb the gas flow over the susceptor are minimized. Gas flow is required only for susceptor sustentation. PA1 (c) Rotation rates of susceptors can be determined essentially exactly. Perfect matching of rotation rates between individual susceptors in the planetary device is an inherent characteristic. Different adjustable rotation rates for different susceptors in the planetary device are also possible. PA1 (d) Failures or rate changes in the susceptor rotation due to external influences can be quickly detected with the optional sensors, thereby allowing for possible corrective actions to be taken by the control electronics or a human operator. PA1 (e) The process of starting and stopping rotation generates no contaminating particles, because sustentation of susceptor is independent of the rotation mechanism. PA1 (f) Synchronization of rotation with external events, such as source-gas switching during an epitaxial growth, is simple. PA1 (g) Relative angular orientation and position of each susceptor are known at almost all times with the optional sensors, thereby allowing for device incorporation into automated processing machines.